For improving the impact resistance of a thermoplastic resin, heretofore widely employed is adding thereto a graft copolymer obtained in an emulsion polymerization method or a suspension polymerization method. For example, it is known to incorporate a diene-based or acrylate-based graft copolymer (e.g., see Patent Reference 1).
However, when a diene-based graft copolymer is added to a thermoplastic resin, then the impact resistance could be improved but the weather resistance is poor, and therefore, in case where the produced molded article is used outdoors, it has a drawback in that its impact resistance greatly worsens. Accordingly, as an impact resistance improver for outdoor use, proposed is an alkyl (meth)acrylate-based graft copolymer for improving the weather resistance of the diene-based one and for imparting the impact resistance thereto (e.g., see Patent Reference 2).
In general, as compared with that of a diene-based rubber, the effect of a (meth)acrylate-based rubber for improving impact resistance is small, and therefore, its amount to be added to a thermoplastic resin must be large. However, in the field of improving the impact resistance of a thermoplastic resin, it is desired to reduce as much as possible the amount of the impact resistance improver, graft copolymer to be added, from the viewpoint of the quality and the cost of products; and investigations for improving this point have been made for a long period of time (e.g., see Patent References 3 and 4).
As a method for improving the impact resistance of a vinyl chloride-based resin, disclosed is a technique of adding to the vinyl chloride-based resin, a graft copolymer containing an acrylic rubber composed of a unit derived from butyl acrylate and a unit derived from an alkyl acrylate of which alkyl group has from 8 to 12 carbon atoms (e.g., see Patent Reference 5). According to the method, the impact resistance could be improved; however, for example, in extrusion of the vinyl chloride-based resin composition, there is a problem in that screw motor load is high, and therefore, it is hard to say that the method is a satisfactory method.
On the other hand, various methods are known for improving the effect of graft copolymer to impart impact resistance to a thermoplastic resin; and among them, it is known that a method of improving the quality and the quantity of the rubbery core in a graft copolymer by lowering the glass transition temperature of the rubbery core in the graft copolymer or by increasing the ratio by weight of the rubber core in the graft copolymer is effective for its object. In particular, it is believed that a method of increasing the ratio by weight of the rubbery core in a graft copolymer to at least 90% by weight and, in that condition, lowering the glass transition temperature of the rubbery core may be effective for imparting high-level impact resistance to the resin.
For example, for obtaining an impact resistance improver in which the ratio by weight of the rubbery core of a graft copolymer is high, there is disclosed a technique of making the polymer of the innermost layer have a specific monomer composition and defining the particle size of the impact resistance improver to fall within a specific range (e.g., see Patent Reference 6). According to this method, the ratio by weight of the rubbery core may be increased; however, since the particle size of the impact resistance improver is limited, there is a problem in that the quality degradation except impact resistance is inevitable. For example, it is known that increasing the particle size of a graft copolymer causes degradation of the physical properties such as typically surface gloss of molded articles. In addition, in case where the particle size of a impact resistance improver in a thermoplastic resin is large, it may be effective for increasing the degree of stress concentration; however, at the same time, it is known that there may occur reduction of the degree of stress concentration owing to prolongation of the interparticle distance; and in particular, in case where the number of the parts of the impact resistance improver to be incorporated is small, the influence of the prolongation of the interparticle distance may be great and there is a problem that the impact resistance-improving effect could not be sufficiently obtained.
Further, in case where the above-mentioned method is employed, the particles themselves may be sticky and therefore, when the graft copolymer particles are collected from an emulsion polymerization latex or a suspension polymerization slurry, they may grow to coarse grains or may form lumps. Even though such a resin is incorporated in a thermoplastic resin, a sufficient impact resistance-improving effect could not be obtained, and further, it may cause appearance failure of molded articles. This is because the impact resistance improver that may readily grow to coarse grains or may readily form lumps could not be uniformly mixed when they are added to and blended with a thermoplastic resin, and further even though the coarsened or lumped impact resistance improver particles are incorporated and processed in a thermoplastic resin, they could not be fully dispersed; and the dispersion failure phenomenon is confirmed through electromicroscopic observation of molded articles. Accordingly, for example, in case where a vinyl chloride-based resin is used as a thermoplastic resin, generally carried out is a step of removing the coarsened or lumped particles by sieving before the impact resistance improver is added thereto and processed.
Therefore, in industrial practice, an impact resistance improver from which coarsened grains are previously removed is used, and it is advantageous in view of the cost thereof to reduce as much as possible the amount of the coarse grains in producing the impact resistance improver; and accordingly, it is said indispensable to restrict the glass transition temperature of the soft polymer phase in a graft copolymer and to restrict the ratio by weight of the soft polymer phase in a graft copolymer.
On the other hand, as a method for collecting a sticky rubbery polymer latex as a poorly-sticky resin powder, there is known a method comprising adding a high-molecular-weight polyanion having carboxyl groups and/or hydroxyl groups in the molecule to a rubber latex, and dropwise adding the mixed latex to an aqueous solution containing at least one alkaline earth metal (e.g., see Patent Reference 7).
According to the method, however, it is said that the high-molecular-weight polyanion must be added in an amount of at least from 2 to 8 parts by weight, preferably from 4 to 6 parts by weight relative to 100 parts by weight of the solid rubber content of the rubber latex, and if not, the stickiness of the collected resin powder could not be prevented. In general, when an impurity (in this case, high-molecular-weight polyanion) in an amount of 4 parts by weight or more is added to a polymer latex, then it may be easily anticipated that the intrinsic quality of a collected polymer composition itself that is used for various purposes may worsen. In particular, in case where the technique is applied to a graft copolymer, of which the amount to be added to a thermoplastic resin and the like for the purpose of imparting impact resistance thereto is desired to be reduced, it is inevitable to worsen the quality such as the impact resistance-imparting effect thereof, and therefore, it is hard to say that the method is a satisfactory method.
In other words, the current situation is that it is continued to expect the development of a thermoplastic resin composition capable of satisfying at a high level, the contradictory requirements of both the improvement of impact resistance and the suppression of processability and quality degradation and cost increase owing to the addition of an impact resistance improver.    Patent Reference 1: Japanese Examined Patent Publication No. 39-19035    Patent Reference 2: Japanese Examined Patent Publication No. 51-28117    Patent Reference 3: Japanese Examined Patent Publication No. 42-22541    Patent Reference 4: Japanese Patent Laid-open Publication No. 2-1763    Patent Reference 5: Japanese Patent Laid-open Publication No. 8-100095    Patent Reference 6: Korean Patent Laid-open Publication No. 2004-62761 (A)    Patent Reference 7: Japanese Patent Laid-open Publication No. 52-37987